Microwave cooking



Jan. 25, 1966 E. z. KRAJr-:wsKl 3,230,864

MICROWAVE COOKING Filed May l5. 1961 y?! ji I cam/ny Mir/aware United,States Patent .3,239,8641 MICROWAVE y,COKING Edward Zigmund KrajewskhBelmont, Calif., assigner to Litton Precision Products, Inc'., a.corporation of Delaware y Filed May 15, 196i., Ser. No. 1f0,120 2vClaims. (Cl. 99-446) This invention relates to microwave cooking, andmore particularly toa method and apparatus for increasing the efficiencyof microwave cooking.

In the field of' microwave cooking, a considerable problem relates tothe loss of eflicieney` resulting from water, meat juices or otherliquids which are released from food heated in a microwave oven. Thesejuices tend to absorb microwave energy. until they are evaporated', and'this evaporation requires the absorption of large amounts of energy.More specifically, the heat of vaporization of water is about 539calories pre gram of water. This means that the evaporation of one unitweighty of liquid requires the same amount of energy as would berequired to raise the temperature of ten times as much liquid, forexample, by more than 50 degrees centigrade.

Accordingly, a principal object of the present invention is toy avoidvthe loss of eiiiciency which. occurswhen juices and other liquids arereleased in the course of microwave heating, without adversely affectingthe cooking process in any other way.

A closely related problem. in the microwave cooking field, isthedefros'ting of frozenv foods such. as strawberries, eggs, peas andthe like. As a portion of the frozeny foods are defrosted and separatedfrom the main mass of the frozen foods, it would be desirable to removethe defrosted portion from the microwave lield. This would have thedesirable effect of'avoiding overcooking the part ofthe foodV which isfirst defrosted. i

Accordingly, another object of the present invention is to permit theremoval from theV heating zone of a microwave oven, of food which hadbeen defrosted and separate it from a mass of frozen food..

In accordance with the present invention, the objects noted above may beachieved through the use of a perforated4 conducting surface whichformsfa boundary for the microwave energy of the oven. In the case ofthe Iliquids mentionedabove, they may readily passv through theperforated sheet or screen and drop into a container below. Similarly,in. the case of a` massof food which separates as itisdefrosted and.cooked to the desired extent, the defrosted portions may drop throughthe perforations into a container which is located below and out of themicrowave field.

An insulating sheet may be provided-` with apertures which match thoseof the conducting sheet. Such an insulating member prevents heat loss tothe metal pan by conduction and permits uniform heating throughout theremainder of the material in the microwave oven through proper spacingfrom the conductive sheet, where the energy level yis low. In the caseof defrosting large masses of material which drop through the screen asthey are defrosted, apparatus may be provided for vibrating theperforated sheet or screen soy that the defrosted material Will morereadily drop through the Screen. By regulation of the microwave energylevel and by delaying drop out from the microwave heating zone, anydesired degree of cooking may be achieved.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of construction andoperation, together with further objects and advantages thereof, will bebetter understood from the following description considered inconnection with the accompanying drawing in which illus- 3,239,864Patented Jan. 25, 1966 ICC trative embodiments of the invention aredisclosed, by way of example. It is to be expressly understood, however,that the drawing is for the purpose of illustration and descriptionlonly andv does not dene limitations of the invention.

In the drawing:

FIG. l shows a conventional microwave oven utilizing a cooking pan inaccordance with the present invention;

FICir.l 2 shows a microwave oven in accordance with the presentinvention in which a perforated conducting sheet or screen is employed;

FIG. 3 shows an insulating cooking sheet having perforations which matchthe perforations ,in the screen of FIG. 2; and

FIG. 4 is a cross-sectional view through the bottom of the sheet of FIG.3.

Referring to the drawing, FIG. 1 shows a microwave oven having sidewalls12 and 14 and a bottom 16. Microwave energy is supplied from a source(not shown) through the waveguide 18 to an aperture 2d in the top wall22 of the microwave oven. Suitable mode mixing and other known adjunctsemployed in microwave heating systems may be used. The rear wall 24 may,for example, be a door through which material may be placed into theoven. The source of microwave energy maybe a magnet-ron power supply ofa type well-known in the industry'.

The present invention relates, in one aspect, to the food supporting pan26. This pan includes a conductive receptacle portion 28 and a coverwhich is made up of a highly conductive sheet portion 29 and aninsulating sheet portion 30. The conductive sheet may be of aluminum,for example, or other good conducting material. The conductive portion29 preferably has depending edges 32 to make good electrical contactwith the receptacle 28. These edges 32 may make a spring lit with thewalls of the receptacle 28 so that the lid is in intimate engagementwith the receptacle, and does not merely loosely rest on it in themanner of conventional cooking utensils. In practice, a roast 34 isplaced on the insulating sheet 30 for cooking in t-he microwave oven. Asthe roast 34 is defrosted andi starts to cook, moisture and meat juicesdrop through the alined openings 36 in the insulating sheet-.30 and theconductive sheet 29. This liquidv 3S is collected in the bottom of pan28.

After it has fallen through the openings 36, the` liquid 38 is out ofthe microwave field and therefore does not absorbV energy. In order toavoid the penetration of the microwave energy into the receptacle 28,both the receptacle 28 and the conductive plate 29 should be highlyconductive, and they should be in intimate engagement along their edges.The depending' edges 32. insure the necessary goodj electrical contact,as noted above. To avoid penetration of the microwave energy through theopenings 36, they must be relatively small, in comparison with thewavelength of the microwave energy transmitted through the waveguide 18.In one representative example, the microwave frequency was approximately245 0 megacycles, and the resulting wavelength-was approximately 21/2.Under these conditions, the openings 36 should be less than one quarterwavelength, and preferably or smaller. In cases where a conductingscreen is employed or where the openings are spaced closer together thanshown in FIG. 1, the openings should have a smaller crosssectionaldimension, such as 1A or less, when a frequency of 2450 megacycles isemployed. In general, it is desirable that the openings do not have anydimension which is greater than 1A of a wavelength, and the greatestdimensi-on should preferably be less than 1/5 or 1A; of a wavelength.When the foregoing conditions are satisfied, little or no microwaveenergy will penetrate the container, and no energy will be lost in theevaporation of fluid 38.

vention in which the oven 42 is provided with two doors 44 and 46. Thesedoors provide access to the chambers 48 and 5t) within the ovenstructure. The two chambers 48 and 50 are separated by a conductivescreen 52. This 4screen is secured to the oven 42 at its edges and isalso supported by rods 54 which extend across the width of the oven. Theopenings in the screen are sutiiciently small so that microwave energysupplied through waveguide 56 will not pass through the screen 52. Onthe otherhand, the mesh of the conductive screen 52 is suflicientlyv bigto permit selected materials, which are defrosted and cooked to thedesired extent, to fall through the holes into the compartment 50 of theoven.

In operation, a large mass 56 of material to be dclfrosted is placed onthe conductive screen SI2. As portions of this mass are def-rosted theydrop thr-ough .the screen 52 and into the pan 518. By way of example,the frozen mass 56 may be peas, whole frozen strawberries, shrimp, eggs,or other frozen materials, and as these materials are defrosted, theywill drop through the screen 52 into p-an 58. The defrosted materialsmay be retained in the microwave eld through the use of an insulatingscreen and insulating tilted .bathe plates which are vibrated, to retainthe defrosted material lwithin the microwave eld for the desired cookingtime period. -When all of the malterial has dropped through the screen52, the pan 518 may be removed through door 56.

To facilitate the separa-tion action, t-he supporting rods 54 areinterconnected by end rods 68 and these rods are `agitated by amechanical apparatus such as the eccentric wheel 62. The eccentric wheel62 may be selectively driven by a motor (not shown).

The insulating tray 64 of FIG. 3 may also be employed in connection withthe oven 42 of FIG. 2. The vinsulating tray 64 includes sidewalls 616and a bottom `68 which is provided with an array of holes 72. The holes7-2 in the bottom of the plastic tray 64 have the same -spacing as theopenings through the conductive screen 52 of FIG. 2. Furthermore, theopenings 72 maybe formed by a piercing operation from the upper surfaceso that the ed-ges of theholes 72 extend downwardly. Through the use ofthis structure, therefore, juices or other liquids which drip throughthe holes 72, will not touch the wires forming the screen 52. When theinsulating tray 6'4 is employed, a roast may be cooked on its surfacewith the advantages disclosed above in connection with FIG. 1.

The insulating sheets preferably maintain the `food to be cooked spacedfrom the conductive boundary of the microwave cooking zone, where theenergy level is relatively low. Itshould therefore be at least /l inchthick and preferably about 1/8 inch thick. FIG. 4 represents across-sectional View through the bottom 68 of the insulating cookingAsheet of FIG. 3. It may be formed like a vcardboard box, with upper andlower sheets 82 and. a

curved box, with upper and lower sheets 82 and a curved spacing sheet86. i Holes 72 pass through -all three layers and have downwardlyextending portions 88, as mentioned above.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of Ithe invention.Numerous other arrangements may be devised aby those skilled in the artwithout departing from the spirit and scope of the invention. Thus, byway of example and not of limitation, the perforated insulating sheetso-f FIGS. 1 and 3 may be formed of disposable plastic material which maybe thrown out after each use. In addition, other mechanical arrangementsmay be provided which perform the electrical and mechanical functionsdescribed above. Accordingly, it -is to be understood that the presentinvention is limited only by the spirit and scope of the appendedclaims.

' What is claimed is:

1. A cooking utensil for use in a microwave oven, comv prising, incombination, a closed conductive pan, an apertured electricallyconductive cover in `intimate conductive engagement with said pan, andan electrically insulating sheet having apertures aligned with theapertures in said the apertures in said conductive cover and insulatingsheet being adapted to support an yobject to be heated in a microwaveoven.

2. A cooking utensil adapted for use in a microwave oven, comprising apan of high conductivity, an apertured electrically conductive coverintimately engaging said pan around -its edges, and an electricallyinsulating sheet mounted above said conductive cover and havingapertures aligned with the apertures in said `conductive cover, theapertures in said conductive cover and insulating sheet having theirlargest dimension less ythan 1A wavelength of the Ifrequency at whichsaid utensil is to be used.'

References Cited by the Examiner UNITED STATES PATENTS 616,883 1/ 1899Bowers 220-44 2,003,494 6/ 1935 Reynolds 229-3.5 2,501,400 3/ 1950Marshall 99-22-1 2,582,174 1/1952 Spencer 99-221 2,593,592.- 4/ 1952Miller 99-446 2,597,825 5/ 1952 Schroeder 219-10.55 X 2,600,566 6/ 1952Moffett Z119-10.55Y X Y 2,612,596 9/19'52 GIOSS V219/-1055 2,714,070 7/1955 Welch 219-10.55 X 2,762,893 9/1956A Long et al. 219-10.55 2,864,93212/1958 Forrer 21935 2,866,551 12/ 1958 Schlebusch 219-10.69 X 2,888,5425/1959 Mork 219-10.55 3,127,828 4/ 1964 Fine 99-446 RICHARD M. wooD,Effmmy Examiner.

LORD, Examiner.

1. A COOKING UTENSIL FOR USE IN A MICROWAVE OVEN, COMPRISING, INCOMBINATION, A CLOSED CONDUCTIVE PAN, AN APERTURE ELECTRICALLYCONDUCTIVE COVER IN INTIMATE CONDUCTIVE ENGAGEMENT WITH SAID PAN, AND ANELECTRICALLY INSULATING SHEET HAVING APERTURES ALIGNED WITH THEAPERTURES IN SAID THE APERTURES IN SAID CONDUCTIVE COVER AND INSULATINGSHEET BEING ADAPTED TO SUPPORT AN OBJECT TO BE HEATED IN A MICROWAVEOVEN.